Non-reciprocal circuit devices such as isolators, etc. are widely used in mobile communications equipment utilizing frequency bands from several hundreds of MHz to ten-odd GHz, such as cell phones and their bases, etc. An isolator is disposed between a power amplifier and an antenna, for instance, in a transmission part of mobile communications equipment, to prevent unnecessary signals from flowing back to the power amplifier and stabilize the impedance of the power amplifier on a load side. Accordingly, the isolator is required to have excellent insertion loss characteristics, reflection loss characteristics and isolation characteristics.
FIG. 27 shows a conventional isolator. This isolator comprises a microwave ferrite 38 made of a ferrimagnetic material, three central conductors 31, 32, 33 disposed on a main surface of the ferrite 38 such that they are crossing at an angle of 120° in a mutually insulated state, matching capacitors C1-C3 each connected to one end of each central conductor 31, 32, 33, and a terminal resistor Rt connected to a port (for instance, P3) of any one of the central conductors 31, 32, 33. The other end of each central conductor 31, 32, 33 is grounded. A DC magnetic field Hdc is applied from a permanent magnet (not shown) to the ferrite 38 in its axial direction. In this isolator, a high-frequency signal input through the port P1 is transmitted to a port P2, and reflected waves from the port 2 are absorbed by the terminal resistor Rt, and therefore not transmitted to the port P1. Thus, unnecessary reflected waves generated by the impedance variations of the antenna are prevented from flowing back to the power amplifier, etc.
Recently proposed is an isolator with a different equivalent circuit from that of the above isolator, which has excellent insertion loss and reflection loss characteristics (JP 2004-88743 A). This isolator having two central conductors is called “two-terminal-pair isolator.” An equivalent circuit of its basic structure is shown in FIG. 24. This two-terminal-pair isolator comprises a first central electrode (first inductance element) L1 disposed between a first input/output port P1 and a second input/output port P2, a second central electrode (second inductance element) L2 disposed between the second input/output port P2 and a ground such that it is crossing the first central electrode L1 in an electrically insulated state, a first capacitance element C1 disposed between the first input/output port P1 and the second input/output port P2 for constituting a first parallel resonance circuit with the first central electrode L1, a resistance element R, and a second capacitance element C2 disposed between the second input/output port P2 and the ground for constituting a second parallel resonance circuit with the second central electrode L2.
A frequency at which isolation (reverse attenuation) is at maximum is set in the first parallel resonance circuit, and a frequency at which insertion loss is at minimum is set in the second parallel resonance circuit. When a high-frequency signal is transmitted from the first input/output port P1 to the second input/output port P2, the first parallel resonance circuit between the first input/output port P1 and the second input/output port P2 is not resonated, but the second parallel resonance circuit is resonated, resulting in small transmission loss (excellent insertion loss characteristics). Current flowing from the second input/output port P2 back to the first input/output port P1 is absorbed by the resistance element R between the first input/output port P1 and the second input/output port P2.
FIG. 25 shows a specific example of the structure of the two-terminal-pair isolator. The two-terminal-pair isolator 1 comprises casings (upper casing 4 and lower casing 8) made of a ferromagnetic metal such as soft iron, etc. for forming a magnetic circuit, a permanent magnet 9, a central conductor assembly 30 comprising a microwave ferrite 20 and central conductors 21, 22, and a laminate substrate 50, on which the central conductor assembly 30 is mounted.
The upper casing 4 for containing the permanent magnet 9 substantially has a box shape having an upper portion 4a and four side portions 4b, and the lower casing 8 has a U-shape having a bottom portion 8a and two side portions 8b, 8b. Each casing 4, 8 is plated with conductive metals such as Ag, Cu, etc.
The central conductor assembly 30 comprises a disk-shaped microwave ferrite 20, and first and second central conductors 21, 22 disposed on an upper surface of the microwave ferrite 20 such that they are perpendicularly crossing each other via an insulation layer (not shown), the first and second central conductors 21, 22 being electromagnetically coupled at a cross. The first and second central conductors 21, 22 are respectively constituted by two strip lines, and both end portions 21a, 21b, 22a, 22b of each line are separate from each other and extend onto a bottom surface of the microwave ferrite 20.
FIG. 26 shows the structure of the laminate substrate 50. The laminate substrate 50 comprises a sheet 46a having electrodes 51-54 connected to the ends of the central conductors 21, 22 on a rear surface, a dielectric sheet 41 having capacitor electrodes 55, 56 and a resistor 27 on a rear surface, a dielectric sheet 42 having a capacitor electrode 57 on a rear surface, a dielectric sheet 43 having a ground electrode 58 on a rear surface, and a dielectric sheet 45 having an input external electrode 14, an output external electrode 15 and ground external electrodes 16, etc.
The central-conductor-connecting electrode 51 corresponds to the first input/output port P1, the central-conductor-connecting electrode 52 corresponds to the third port P3, and the central-conductor-connecting electrodes 53, 54 correspond to the second input/output port P2 in the above equivalent circuit. One end 21a of the first central conductor 21 is connected to the input external electrode 14 via the first input/output port P1 (central-conductor-connecting electrode 51). The other end 21b of the first central conductor 21 is connected to the output external electrode 15 via the second input/output port P2 (central-conductor-connecting electrode 54). One end 22a of the second central conductor 22 is connected to the output external electrode 15 via the second input/output port P2 (central-conductor-connecting electrode 53). The other end 22b of the second central conductor 22 is connected to the ground external electrode 16 via the third port P3 (central-conductor-connecting electrode 52). The first capacitance element C1 (25) is connected between the first input/output port P1 and the second input/output port P2, to form the first parallel resonance circuit with the first central conductor L1 (21). The second capacitance element C2 (26) is connected between the second input/output port P2 and the third port P3, to form the second parallel resonance circuit with the second central conductor L2 (22).
To obtain a non-reciprocal circuit device having excellent electric characteristics, various factors providing inductance generated by lines connecting reactance elements, floating capacitance generated by interference between electrode patterns, etc., should be taken into consideration.
It is likely in the above two-terminal-pair isolator that unnecessary reactance components are connected to the first and second parallel resonance circuits. If that happens, the input impedance of the two-terminal-pair isolator is deviated from a desired level, resulting in impedance mismatching with other circuits connected to the two-terminal-pair isolator, and thus the deterioration of insertion loss characteristics and isolation characteristics.
Though the inductance and capacitance of the first and second parallel resonance circuits can be determined by taking unnecessary reactance components into consideration, simple changing of the width and gap, etc. of lines constituting the first and second central conductors 21, 22 would fail to obtain optimum matching conditions with external circuits. This is because the mutual coupling of the first and second central conductors 21, 22 changes the inductance of the first and second inductance elements L1, L2, resulting in difficulty in independently adjusting input impedance at the first and second input/output ports P2, P1. Particularly the deviation of input impedance at the first input/output port P1 should be prevented because it leads to increase in insertion loss.